This invention relates to a carbon monoxide detecting device having a plurality of semiconductor films.
With respect to conventional gas detecting devices which utilize semiconductor films, a hydrogen or reductive gas detecting device has been proposed, for example, by U.S. Pat. No. 3,479,257.
The detecting device disclosed in U.S. Pat. No. 3,479,257 is shown in FIGS. 1A and 1B. In the detecting device, a pair of electrodes 2 and 3 are provided on a first surface of a substrate 1, while a film 4 of conductive material employed as a resistance for heating is deposited on the opposite surface of the substrate 1 by vacuum evaporation. A metal oxide film 5 is formed between the electrodes 2 and 3, and an activation catalyst 6 is deposited in the form of minute islands on the film 5 by vacuum evaporation. The electrodes 2 and 3 are connected to lead wires 7 and 8, respectively.
The film 5 is a metal oxide selected from the group consisting of tungsten (W), molybdenum (Mo), chromium (Cr), niobium (Nb), nickel (Ni), iron (Fe) and titanium (Ti) or their compounds.
The catalyst 6 is a metal selected from the group consisting of platinum (Pt), iridium (Ir), rhodium (Rh), gold (Au) and palladium (Pd) or their mixtures.
FIG. 2 is a graph plotting in a log/log manner film current in mA on the vertical axis versus hydrogen concentration in PPM on the horizontal axis of the gas detecting device constructed in accordance with the description presented above. As is apparent from FIG. 2, as the hydrogen concentration is increased, the film current I of the metal oxide film is monotonously and gradually increased. In general, a gas detecting device is provided with a warning device which is activated when the detected gas concentration reaches, for instance, 100-300 PPM. Because the film current I is monotonously increased, the rate of change of current I with respect to the rate of change of the detected hydrogen concentration is low in the conventional gas detecting device, and therefore, the setting of the film current to operate the warning device is rather difficult. In addition, the above described patent disclosure is silent as to the sensitivity of the conventional gas detecting device for detection of carbon monoxide.
When a burner is operating in the incomplete combustion state, the burner generates carbon monoxide (CO), but also generates nitrogen oxides (NO.sub.x). However, according to experiments performed by the inventors, the detection characteristic of the conventional gas detecting device is affected by the nitrogen oxides. Specifically, the output characteristic of the carbon monoxide detecting device, having a film current characteristic which monotonously increases, is lowered by the nitrogen oxides. Therefore, because of the nitrogen oxides, it is very difficult to detect carbon monoxide alone with the carbon monoxide detecting device.